1. Field of the Invention
The present invention relates to water-based ink compositions used for ink-jet printers, stationery, various printing processes, and others.
2. Description of the Related Art
Ink-jet recording is employed as a method for outputting images formed by personal computers or the like onto recording media such as paper or OHP film.
In ink-jet recording, ink droplets are ejected from a nozzle of an ink-jet printer toward a recording medium while using an electrical field, heat, pressure or the like as a driving source, and thus an image is formed on the recording medium.
Such ink-jet recording is advantageous in that it can be performed with low-level noise at a low running cost, images can be formed on plain paper, and no waste such as ink ribbons and toner cartridges is generated. Additionally, if a water-based ink is used, ink-jet recording will be further advantageous since organic solvents evaporating into the air can be dispensed with or reduced. Due to such advantages, in recent years, ink-jet recording apparatuses have rapidly spread for office and personal (family) use.
Incidentally, inks used in, for example, ink-jet printers, require the following properties.
(1) Not to clog the printer head. PA1 (2) To readily penetrate into the recording sheet. PA1 (3) To dry rapidly. PA1 (4) To have superior storage-stability.
These properties depend on the composition of the ink, and the solution physical-properties determined in accordance with the composition.
For example, ink wettability closely concerns the above property (1), surface tension closely concerns the property (2), and drying property closely concerns the property (3), respectively. Satisfactory wettability is necessary for stable ink-ejection from a nozzle of a printer head under any conditions. Surface tension closely relates to the image quality, namely, directional accuracy of ejected ink-droplets, dot formativity, penetrability into recording sheets, generation of satellites, and others.
In order to satisfy the above-described ink properties, a well-balanced selection is required of the solvent and additive in the water-based ink composition. In addition to water, dye-solubilizers are used as solvents. As additives, wetting agents, surfactants, pour point depressants, defoaming agents, antibacterial agents and others are used.
Hitherto, addition of a lower aliphatic monohydric alcohol such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, s-butyl alcohol, and t-butyl alcohol is employed as a method for adjusting the surface tension of an ink for ink-jet recording to a value appropriate for ink-ejection from a nozzle of a printer head. Water-based inks with the addition of alcohols are also frequently adopted as inks for other uses such as writing tools.
Conventional water-based inks contain such solvents and additives, namely, they are mixtures comprising 10 or more ingredients including water.
Due to this, the ingredients mutually interact to influence the solvent physical-properties, the optimum composition balance cannot be easily determined, and actually, an ink composition which sufficiently satisfies the above-described properties cannot be readily achieved.
Further, bleeding, feathering, and tone irregularity are problems inherent in ink-jet recording on plain paper, which has no specific ink-receiving layer. Particularly, in complex characters and smaller characters, bleeding and feathering become further apparent, and therefore, such characters become unrecognizable.
As a remedy for this, a highly-viscous solvent (resin) is further added to the ink to inhibit migration of the coloring material, and thus, bleeding and feathering are prevented.
However, although bleeding and feathering can be inhibited by making the ink-viscosity high, a considerable amount of a viscous solvent should be added for such inhibition. As a result, the ink-viscosity becomes too high, which causes reduction in speed and amount of the ink ejected from the nozzle.
Further, even if a high ink-viscosity can be achieved, the ink may readily spread on plain paper, and may contrarily cause bleeding and feathering when surface tension is severely reduced.